@LUCILOSER used this term in a comment on a post of mine, so I thought I really should do a blog to explain what it means.
TERF is an acronym for Trans-Exclusionary Radical Feminist, or bastard git¹ for short. The term was coined - or at least popularised - in 2008 by Viv Smythe², a trans-inclusive cisgender radical feminist blogger. Originally applied to radical feminists who hold views that other feminists consider transphobic, such as the rejection of trans women as women, the exclusion of trans women from women's spaces, and opposition to transgender rights legislation, usage has since expanded to refer more broadly to people with trans-exclusive views who may have no involvement with radical feminism (bloody English loony etymology being born again).
Cristan Williams defines the term as referencing "a brand of 'radical feminism' that is so rooted in sex essentialism and its resulting biologism, it actively campaigns against the existence, equality, and/or inclusion of trans people.” [Williams, Cristan (24 September 2013). "YOU MIGHT BE A TERF IF ..." The TransAdvocate]
Unsurprisingly perhaps, people referred to as TERF tend to consider it a slur. Well I hope we all know how much I care about that.
¹You know I make this up, right?
²She did this in blog post she wrote reacting to the Michigan Womyn's Music Festival's policy of denying admittance to trans women.